This invention relates to direct-positive silver halide photographic sensitive materials of the lith-type capable of forming a high-contrast image when processed in a lith-developer.
Generally in the field of graphic arts, high-contrast photographic sensitive materials are employed in photographic formation of line images and halftone images (dot images). The image is produced by exposing a lith-type photographic sensitive material suitable for the purpose to a continuous tone original through a screen and developing in a lith-developer containing hydroquinone as single developing agent. In this way, the positive original is reproduced in a negative image which is used in making printing plates by a variety of processes. In one of such processes, the negative image is reproduced again in positive image. Such a process, therefore, requires two steps of treatment for the reproduction of positive image from a positive original by way of intermediary negative reproduction. In recent years, there has been an increasing demand for reproducing a positive image from a positive original in one step. For the purpose of reproducing a positive image from a positive original or reproducing a negative image from a negative original, direct-positive silver halide sensitive materials such as duplicating film have heretofore been employed. When treated in a lith-developer, however, conventional direct-positive silver halide sensitive materials give an image of lower contrast as compared with commercial lith-type photographic sensitive materials so that it was difficult to form a line image and halftone image by using the conventional direct-positive sensitive materials alone, although these sensitive materials are able to reproduce the image reproduced with the lith-type photographic sensitive material.
In a high-sensitivity direct positive silver halide photographic sensitive materials, there are used silver halide emulsions containing silver bromide or silver iodobromide as major constituent. As described in Japanese patent application Laid-Open ("Kokai") No. 69,330/74, such silver halide sensitive materials containing silver bromide or silver iodobromide as major constituent form a low-contrast image upon development in conventionally known lith-developer and, hence, have never been employed for the purpose of producing a high-contrast image. The said patent application disclosed that the above difficulty can be overcome by improving the processing procedure, but no mention was made about the direct-positive silver halide photographic sensitive material.
On the other hand, silver chlorobromide or silver chloroiodobromide emulsions containing major portion of silver chloride have been used in commercial lith-type photographic sensitive materials. Such emulsions have been improved in sensitivity by spectral sensitization. The spectral sensitizers for use in this case must meet the following requirements:
(1) it should not interfere with the photographic feature of especially high contrast,
(2) it should not deteriorate the sharpness of halftone image and not interfere with the improvement in quality of the image in small dot, and
(3) it should not accompany the formation of color stain.
For such reasons, sensitizing dyes such as merocyanines, cyanines of the intramolecular salt type and cyanines of the anion type have been preferred. Further, joint use of such dyes has been disclosed in Japanese patent application Laid-Open ("Kokai") No. 28,224/73. However, no disclosure has ever been made about the high-sensitivity direct-positive silver halide photographic materials containing silver bromide or silver iodobromide as major sensitive material.